Poposaurus walked like a dinosaur. That’s weird, because the Triassic critter wasn’t a dinosaur at all, but a peculiar archosaur more closely related to crocodiles. Even though paleontologists traditionally thought that dinosaurs had an immediate edge over all over forms of Triassic life thanks to muscular legs held close together right beneath their bodies, rather than sprawled out to the sides, there were also croc copycats that cramped dinosaurian style.

Two years ago, I wrote a short ScienceNOW piece aboutPoposaurus and a monograph describing the creature’s postcranial skeleton. Even though the 225 million year old archosaur was named almost a century ago, it has only been recently that more complete individuals have shown how strange Poposaurus truly was.

In the big picture of archosaur evolution, Poposaurus was a pseudosuchian – a group solely represented by alligators, crocodiles, and gharials today, but encompassed a surprising and frightening array of related lineages in the deep past. Roughly, the pseudosuchians are “croc-line archosaurs.” The group not only includes scattered modern species, but totally extinct groups such as the “armadillodile” aetosaurs, terrifying rauisuchians, and the strange lineage of animals that Poposaurus belonged to, called poposauroids, which included bipedal herbivores and sail-backed carnivores such as Arizonasaurus in their ranks.

Poposaurus wasn’t the only one of its kind to strut upright. The first poposauroids discovered to walk like dinosaurs were toothless, beaked herbivores. Effigia, named by paleontologists Sterling Nesbitt and Mark Norell in 2006, was a fortuitous find that outlined the shape of bipedal crocodile cousins. After that, other strange pseudosuchian fossils started to make sense. Shuvosaurus, originally proposed to be a Triassic dinosaur similar to Ornithomimus, turned out to be yet another beak-bearing pseudosuchian with a posture just like that of Effigia.

Evolution spawned anatomical convergence between dinosaurs and pseudosuchians, and, for reasons still unknown, dinosaurs succeeded while the bipedal pseudosuchians died out. The bipedal croc-line archosaurs have made one thing clear, though. Contrary to previous proposals, dinosaurs did not persist and proliferate by virtue of their posture alone.

Effigia and Shuvosaurus were beaked herbivores. What Poposaurus ate, however, has been harder to outline. What little we know of the pseudosuchian’s skull has traditionally come from hard-to-identify fragments. Based upon skull fragments found with some postcranial bones in Arizona’s Petrified Forest National Park, however, paleontologists William Parker and Sterling Nesbitt have been able to confirm that Poposaurus was a roughly 13-foot-long carnivore that retained the sharp, recurved teeth of its ancestors.

After studying a portion of the left upper jaw and a pair of pieces from the lower jaws, the paleontologists found that Poposaurus was a hypercarnivore. Therefore, Parker and Nesbitt note, herbivory and bipedalism did not evolve at the same time among the weird dinosaur mimics. If anything, the various poposauroids were Triassic oddballs who embodied a mix of postures, diets, and even ornamentation during their evolution.

A reconstruction of Postosuchus. Photo by Dallas Krentzel, image from Wikipedia.

Rauisuchids - similar to the fearsome Postosuchus - are the probable Chirotherium trackmakers.

But there was another bipedal pseudosuchian that was even scarier than Poposaurus. Named in 1985, and initially confused for a Tyrannosaurus ancestor (which tells you something about the animal’s fearsome appearance), Postosuchus has since been regarded as an awful apex predator of Late Triassic North America. About as long as Poposaurus, the bulkier Postosuchus had a deeper skull set with teeth that the carnivore likely put to work on aetosaurs and other armored prey of its era. Most restorations envision the hunter clambering through Triassic forests on all fours, but a rexamination of the pseudosuchian’s skeleton conducted by Jonathan Weinbaum hints that Postosuchus might have been a principally bipedal animal.

Postosuchus on all fours has always looked a bit odd to me. Despite being cast as a quadrupedal carnivore, the creature’s forelimbs are relatively small and, as Weinbaum describes them, “slender.” The hindlimbs of Postosuchus, by comparison, were more stoutly constructed, with a large, broad foot. The body of Postosuchus looks as if the animal could have easily balanced on two legs, and, as Weinbaum suggests, the carnivore may very well have been a habitual biped. Weinbaum acknowledges that the hypothesis needs additional testing, but there’s no reason to take the traditional, four-on-the-floor reconstruction of Postosuchus as a given.

Postosuchus as a biped. Image courtesy Jonathan Weinbaum.

Postosuchus envisioned in a bipedal pose.

Despite being a predator of similar size and posture, though, Postosuchuswasn’t a close relative of Poposaurus. As Weinbaum points out, Postosuchus belonged to a tangle of pseudosuchians currently categorized as “rauisuchids” – principally predatory croc cousins that were the major carnivores of their time. So if Postosuchus really did walk on two legs, then bipedal crocodile cousins evolved more than once during the Triassic. Think about that for a moment. What if, by some quirk of history, the relatively marginal dinosaurs of the Late Triassic were all wiped out, and the pseudosuchians survived. What kinds of unseen, dinosaur-like forms would have evolved? Or would the pseudosuchians have kicked off an entirely different, even stranger profusion of Mesozoic life? We’ll never know. Poposaurus, Postosuchus, and their kind offer a tantalizing, yet frustrating, glimpse at evolutionary threads that were cut short at the end of the Triassic.

Related

I dunno, man. I think Postosuchus as a biped looks just as strange–if not stranger. It looks a bit like short-legged Majungasaurus, with too much weight in front of the hips and not enough behind. The neural spines must have impeded upward mobility of the spine to “push back” the center of gravity. Damn weird if true, though.

Zach: Hopefully we’ll see some biomechanics tests for Postosuchus in the future. And even for Poposaurus – even though some pseudosuchians mimicked dinosaurs in a rough sense, they clearly were different in the details.

Jerrold: Prestosuchus is another “rauisuchid”, closer to Postosuchus than Poposaurus. It’ll be interesting to see where all the various “rauisuchids” eventually fall out (and I’ll be happy to stop using quotes around their name, which makes me sound like I’m being sarcastic).

Isn’t that a coincidence? Me and my academic advisor Jonathan Weinbaum were just talking the Rauisuchia paper that came out and we even sent me the online version. For me and Weinbaum, the bipedal posture sure is accurate enough for us.

Rany Irmis’ recent UFOP talk had a great takeaway lesson. Modern crocodilians are not representative of their Triassic forebears, and there are recent fossils that show that crocodylomorphs were even more diverse earlier than previously understood.

Imagine the first prehistoric findings…they all seemed weird and several nary seemed possible. Impressive data shows that biped creatures were not only a possibility but a reality. Weinbaum is brilliant.

I believe that even though life on earth in this day and age is very refined, back in the prehistoric times is where i’d like to be. The lush trees, the inviting forest. Creatures of exceptional ability such as this. It makes you wonder what could have been, dosen’t it?

What is it about Poposaurus and poposaurids that make paleontologists think they were croc relatives? The calcaneal tuber? The large reptile tree at reptile evolution.com recovers poposaurids as phytodinosaurs with a convergent development of a calcaneal tuber, the only dinosaurs to do so. This is convergent with crocs themselves, which also developed a calcaneal tuber after basal crocs, like Gracilisuchus and Scleromochlus, did not have one. One trait, even an ankle trait, does not trump the suite. It’s time to add more pertinent taxa and rerun the analysis.

Who We Are

Phenomena is a gathering of spirited science writers who take delight in the new, the strange, the beautiful and awe-inspiring details of our world. Phenomena is hosted by National Geographic magazine, which invites you to join the conversation. Follow on Twitter at @natgeoscience.

Ed Yong is an award-winning British science writer. Not Exactly Rocket Science is his hub for talking about the awe-inspiring, beautiful and quirky world of science to as many people as possible.
Follow @edyong209
Subscribe via RSS